lib: Create a Counter object

With increment() and decrement() functions that can be used to change
the GtkAdjustment's value. These functions return None if the value did
not change.

Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>

lib/counter.py

@@ -0,0 +1,22 @@
+# Copyright 2021 (c) Anna Schumaker.
+from gi.repository import Gtk
+
+class Counter(Gtk.Adjustment):
+    def __init__(self, min, max):
+        Gtk.Adjustment.__init__(self)
+        self.configure(value=min, lower=min, upper=max + 1, step_increment=1,
+                                  page_increment=1, page_size=1)
+
+    def __change_value__(self, n):
+        value = self.get_value()
+        self.set_value(value + n)
+
+        if self.get_value() == value:
+            return None
+        return self.get_value()
+
+    def increment(self):
+        return self.__change_value__(1)
+
+    def decrement(self):
+        return self.__change_value__(-1)

lib/test_counter.py

@@ -0,0 +1,20 @@
+# Copyright 2021 (c) Anna Schumaker.
+from . import counter
+from gi.repository import Gtk
+import unittest
+
+class TestCounter(unittest.TestCase):
+    def test_counter(self):
+        c = counter.Counter(1, 10)
+
+        self.assertIsInstance(c, Gtk.Adjustment)
+
+        self.assertEqual(c.get_lower(), 1)
+        self.assertEqual(c.get_upper(), 11)
+        self.assertEqual(c.get_value(), 1)
+
+        for i in [ 2, 3, 4, 5, 6, 7, 8, 9, 10, None ]:
+            self.assertEqual(c.increment(), i)
+
+        for i in [ 9, 8, 7, 6, 5, 4, 3, 2, 1, None ]:
+            self.assertEqual(c.decrement(), i)